This invention relates to power supplies for illumination systems.
In recent years, new forms of lighting including low-voltage halogen lamps and gas discharge lamps such as compact fluorescent and high intensity discharge lamps (or HID lamps including metal-halide and sodium lamps) have become increasingly popular owing to their superior efficiency and light color. Unlike conventional incandescent lamps which can be powered directly from the 120V/60 Hz or 230V/50 Hz utility power, these lamps require power supplies. Specifically, low-voltage halogen lamps require a transformer to provide a voltage typically equal to 12V and gas-discharge lamps require an ignition mechanism and a ballast to control the currents running through them.
With the increased popularity of these types of lamps, it is becoming increasingly important to find economical and aesthetic ways of providing for their power needs. It is also desirable to provide more versatile power supply systems which allow consumers to mix different types of lamps together economically and aesthetically, in a manner not hitherto allowed for.
In this context it is important to note that all known approaches to powering modem lamps involve having a single power supply for each lamp (with the limited exception that identical low-voltage halogen lamps can be connected in parallel to a single transformer in an arrangement known as a low-voltage lighting track) such arrangement necessarily being costly and anesthetic in that individual power supplies are bulky and expensive.
It is known in the art that the transformer for a low-voltage lamp may be replaced by a small ferrite based transformer if the input voltage passes through an electronic inverter which produces a square-wave voltage of high frequency, typically about 30 KHz.
It is also known that a ballast for a gas discharge lamp, in which the central component is typically an inductance, can be made smaller by using electronic circuits switching at a high frequency again typically of the order of 30 KHz.
In particular, the approach of inverting 50 Hz or 60 Hz utility power to give high frequency current of 30 KHz modulated at 50 Hz or 60 Hz has been thought inapplicable to HID lamps because the arc in the HID lamps is likely to extinguish at the zero-crossing of the envelope due to the fact that the amplitude of the high frequency alternating voltage becomes very low for a number of milliseconds. Thus, there us up to now been no practical way to unify any elements of the power supplies for halogen and HID even had the concept of a central unit with some common elements been conceived.
In addition to the apparent lack of compatibility of the approaches to miniaturizing power supplies for halogen and HID, the use of high frequency for even systems of one type of lamp is subject to a drawback: namely that the square wave 30 KHz used in power supplies for lighting necessarily contains strong harmonics of much higher frequencies than 30 KHz. When the power supply is not adjacent to the lamp, the wires connecting the two act as a transmission line emitting electromagnetic radiation which can interfere with surrounding equipment and which may violate European, FCC or equivalent standards for electromagnetic compatibility. Clearly this drawback becomes far more serious as the power is increased and as the illumination system extends over larger distances. In practice, this places a limitation on the number of lamps which may be connected simultaneously to the system.
A low-voltage lighting track operating at 12V is known which is specifically designed for low-voltage halogen lights and which is sometimes powered by a so-called electronic transformer which includes a central inverter in combination with a central transformer. Such a system suffers from the problem described above and this is generally overcome by limiting, the length of the system, particularly in Europe, to about two meters, and by limiting the current to about 20 amps or 25 amps, so as to limit the magnitude of the electromagnetic radiation emanating from the system. Clearly, this system cannot be used with lamps other than low voltage lamps.
It is therefore an object of the invention to power economically and aesthetically lighting systems containing mixed types of lamps (line-voltage incandescent, low-voltage incandescent, fluorescent, compact fluorescent and high intensity discharge) and/or mixed types of fixtures (track, recessed etc.) by having one central power supply circuit performing a number of functions which are relevant to all lamps while having secondary power supply systems which are relatively very small and very cheap adjacent to individual lamps.
One key function which may be achieved centrally according to the invention is the inversion of the utility power to a current of a much higher frequency.
One key aspect of the invention is an innovative approach to a ballast for HID lamps which is able to work with a central source of high frequency current even though the current may be modulated by a rectified 50 Hz or 60 Hz envelope. This is achieved by using higher voltages than is customary or by using an energy storage device (valley fill) to store energy for releasing to the lamp in order to preserve the arc at times around the zero crossing of the modulating envelope.
Another key aspect of the invention is an innovative approach to producing high frequency current which is not a square wave but rather has weaker harmonics than a square wave or, in one embodiment in which an inductance and a capacitance in the central power supply together with the external load form a resonant circuit, is virtually sinusoidal therefore reducing any problems of radio interference. Further, one of the ideas according to the invention is to keep the RMS voltage emanating from the central power supply substantially higher than 12V which is the value customary in the only high frequency system in use today (the so called low-voltage lighting track which when powered with a so called electronic transformer) therefore allowing far smaller currents to be used thereby further reducing the radio emissions and also reducing ohmic losses. In particular, these innovations allows the conductors carrying the power to the fixtures to be tens of meters in length compared to the two meters accepted in low-voltage lighting tracks, particularly in Europe, and allow the system to carry hundreds or a few thousand watts of power compared to about 250 W which is a common value in existing systems.
It is a further object of the invention to give better performance and further economies by optionally centralizing functions including the power factor correction, valley fill, supply of low-voltage power (typically 3V) for electrode heating of compact fluorescent lamps, protection circuits, high frequency filters and voltage stabilization.
According to a broad aspect of the invention there is provided an illumination system comprising:
a power supply circuit having an input for connecting to a voltage source of low frequency for providing an output voltage with altered electrical characteristics,
a pair of conductors coupled to an output of the power supply circuit, and
a first lamp coupled to the conductors via a second power supply circuit, and
at least one further lamp with electrical power requirements of a different characteristic to the first lamp coupled to said conductors.